1. Field of the Invention
The invention relates generally to the detection of liquid in a pipeline. More particularly, embodiments of the invention relate to the detection of stratified flow in a pipeline. An embodiment of the invention detects the presence and volume of stratified flow in a pipeline based on time of flight or velocity of sound measurements for an ultrasonic meter.
2. Description of the Related Art
After a hydrocarbon, such as natural gas, has been removed from the ground, it is commonly transported from place to place via pipelines. Often this gas stream also contains a certain amount, or percent fraction, of liquid. As is appreciated by those of skill in the art, it is desirable to know with accuracy the amount of gas in the gas stream. It is also extremely desirable to know whether liquid is being transported along with the gas stream. For example, the presence of a stratified flow of liquid in the gas stream may indicate a production problem upstream of the measurement device. A xe2x80x9cstratified flowxe2x80x9d of liquid consists of a stream or river traveling along one area of the pipeline, such as the bottom. If the measurement device is at the location where the gas is changing hands or custody, and if the gas contains xe2x80x9cnatural gas liquidsxe2x80x9d or condensates, a seller of gas wants extra compensation for this energy-rich liquid.
Gas flow meters have been developed to determine how much gas is flowing through the pipeline. One type of meter to measure gas flow is called an ultrasonic flow meter. Ultrasonic flow meters are also named sonic or acoustic flow meters.
FIG. 1A shows an ultrasonic meter suitable for measuring gas flow. Spoolpiece 100, suitable for placement between sections of gas pipeline, has a predetermined size and thus defines a measurement section. A pair of transducers 120 and 130, and their respective housings 125 and 135, are located along the length of spoolpiece 100. A path 110, sometimes referred to as a xe2x80x9cchordxe2x80x9d exists between transducers 120 and 130 at an angle xcex8 to a centerline 105. The position of transducers 120 and 130 may be defined by this angle, or may be defined by a first length L measured between transducers 120 and 130, a second length X corresponding to the axial distance between points 140 and 145, and a third length D corresponding to the pipe diameter. Distances X, D and L are precisely determined during meter fabrication. Points 140 and 145 define the locations where acoustic signals generated by transducers 120 and 130 enter and leave gas flowing through the spoolpiece 100 (i.e. the entrance to the spoolpiece bore). In most instances, meter transducers, such as 120 and 130, are placed a specific distance from points 140 and 145, respectively, regardless of meter size (i.e. spoolpiece size). A fluid, typically natural gas, flows in a direction 150 with a velocity profile 152. Velocity vectors 153-158 indicate that the gas velocity through spool piece 100 increases as centerline 105 of spoolpiece 100 is approached.
Transducers 120 and 130 are ultrasonic transceivers, meaning that they both generate and receive ultrasonic signals. xe2x80x9cUltrasonicxe2x80x9d in this context refers to frequencies above about 20 kilohertz. Typically, these signals are generated and received by a piezoelectric element in each transducer. Initially, D (downstream) transducer 120 generates an ultrasonic signal that is then received at, and detected by, U (upstream) transducer 130. Some time later, U transducer 130 generates a reciprocal ultrasonic signal that is subsequently received at and detected by D transducer 120. Thus, U and D transducers 130 and 120 play xe2x80x9cpitch and catchxe2x80x9d with ultrasonic signals 115 along chordal path 110. During operation, this sequence may occur thousands of times per minute.
The transit time of the ultrasonic wave 115 between transducers U 130 and D 120 depends in part upon whether the ultrasonic signal 115 is traveling upstream or downstream with respect to the flowing gas. The transit time for an ultrasonic signal traveling downstream (i.e. in the same direction as the flow) is less than its transit time when traveling upstream (i.e. against the flow). The upstream and downstream transit times can be used to calculate the average velocity along the signal path. In particular, the transit time t1, of an ultrasonic signal traveling against the fluid flow and the transit time t2 of an ultrasonic signal travelling with the fluid flow may be defined:                               t          1                =                  L                      c            -                          V              ⁢                              x                L                                                                        (        1        )                                          t          2                =                  L                      c            +                          V              ⁢                              x                L                                                                        (        2        )            
where,
c=speed of sound in the fluid flow;
V=average axial velocity of the fluid flow over the chordal path in the axial direction;
L=acoustic path length;
x=axial component of L within the meter bore;
t1=transmit time of the ultrasonic signal against the fluid flow; and
t2=transit time of the ultrasonic signal with the fluid flow.
The upstream and downstream transit times can be used to calculate the average velocity along the signal path by the equation:                     V        =                                                            L                2                                                              2                  ⁢                  x                                ⁢                                  xe2x80x83                                                      ⁢                          t              1                                -                                    t              2                                                      t                1                            ⁢                              t                2                                                                        (        3        )            
with the variables being defined as above.
The upstream and downstream travel times may also be used to calculate the speed of sound in the fluid flow according to the equation:                     c        =                                                            L                ⁢                                  xe2x80x83                                                            2                ⁢                                  xe2x80x83                                                      ⁢                          t              1                                +                                    t              2                                                      t                1                            ⁢                              t                2                                                                        (        4        )            
Given the cross-section measurements of the meter carrying the gas, the average velocity over the area of the gas may be used to find the quantity of gas flowing through spoolpiece 100. Typically, these measurements are based on a batch of ten to thirty ultrasonic signals rather than upon only one upstream and downstream signal. Alternately, a meter may be designed to attach to a pipeline section by, for example, hot tapping, so that the pipeline dimensions instead of spoolpiece dimensions are used to determine the average velocity of the flowing gas.
In addition, ultrasonic gas flow meters can have one or more paths. Single-path meters typically include a pair of transducers that projects ultrasonic waves over a single path across the axis (i.e. center) of spoolpiece 100. In addition to the advantages provided by single-path ultrasonic meters, ultrasonic meters having more than one path have other advantages. These advantages make multi-path ultrasonic meters desirable for custody transfer applications where accuracy and reliability are crucial.
Referring now to FIG. 1B, a multi-path ultrasonic meter is shown. Spool piece 100 includes four chordal paths A, B, C, and D at varying levels through the gas flow. Each chordal path A-D corresponds to two transceivers behaving alternately as transmitter and receiver. Also shown is an electronics module 160, which acquires and processes the data from the four chordal paths A-D. This arrangement is described in U.S. Pat. No. 4,646,575, the teachings of which are hereby incorporated by reference. Hidden from view in FIG. 1B are the four pairs of transducers that correspond to chordal paths A-D.
The precise arrangement of the four pairs of transducers may be more easily understood by reference to FIG. 1C. Four pairs of transducer ports are mounted on spool piece 100. Each of these pairs of transducer ports corresponds to a single chordal path of FIG. 1B. A first pair of transducer ports 125 and 135 including transducers 120 and 130 is mounted at a non-perpendicular angle xcex8 to centerline 105 of spool piece 100. Another pair of transducer ports 165 and 175 including associated transducers is mounted so that its chordal path loosely forms an xe2x80x9cXxe2x80x9d with respect to the chordal path of transducer ports 125 and 135. Similarly, transducer ports 185 and 195 are placed parallel to transducer ports 165 and 175 but at a different xe2x80x9clevelxe2x80x9d (i.e. a different radial position in the pipe or meter spoolpiece). Not explicitly shown in FIG. 1C is a fourth pair of transducers and transducer ports. Taking FIGS. 1B and 1C together, the pairs of transducers are arranged such that the upper two pairs of transducers corresponding to chords A and B form an X and the lower two pairs of transducers corresponding to chords C and D also form an X.
Referring now to FIG. 1B, the flow velocity of the gas may be determined at each chord A-D to obtain chordal flow velocities. To obtain an average flow velocity over the entire pipe, the chordal flow velocities are multiplied by a set of predetermined constants. Such constants are well known and were determined theoretically.
This four-path configuration has been found to be highly accurate and cost effective. Nonetheless, other ultrasonic meter designs are known. For example, other ultrasonic meters employ reflective chordal paths, also known as xe2x80x9cbouncexe2x80x9d paths.
A pipeline may carry liquid in addition to the gas stream. Liquid level detectors are known that detect whether liquid is present at a location of interest, although typically these liquid level detectors are not positioned inside a pipeline.
One known design of ultrasonic meter is disclosed in U.S. Pat. No. 5,719,329 to Jepson. In particular, this patent discloses a multiphase flow meter that measures film heights of the fluids flowing though the meter by a very complicated scheme using the densities of the mediums in the meter, the pressure of the transmitted wave to the incident wave, and the velocity of sound in the medium. Unfortunately, this multiphase meter is likely too complicated for use in real world applications. The disclosure of the patent also includes a method to confirm the calculated film height by reflection of an ultrasonic signal from, and back to, a single transducer located on the bottom of the meter bore. Unfortunately, however, this method to determine film level is not very accurate (which may be why it is used only as a confirmation and not by itself).
Therefore, a meter or device is needed that is capable of detecting liquid in a pipeline. This device might also measure the amount of stratified flow in a gas stream. The device would be both simple and accurate enough to be used in real-world applications.
Disclosed embodiments of the invention include a method to determine the level of stratified flow in a conduit such as a pipeline, including transmitting an ultrasonic signal through a medium from a first transducer, reflecting the ultrasonic signal from the surface of the stratified flow, receiving the ultrasonic signal at a second transducer, and computing the speed of sound for the ultrasonic signal through the medium (the speed of sound may also be based on a batch of measurements along this same chord). A second speed of sound is also computed based on other ultrasonic signals. A difference in these two computed speeds of sound indicates the presence of stratified flow. Analysis of the difference in these two speeds of sound indicates the level of stratified flow in the pipeline.
The present invention comprises a combination of features and advantages that enable it to overcome various problems of prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.